The read characteristic of a NAND flash memory is mainly rate-determined by charge of bit lines. In such a NAND flash memory, the decrease of memory cell currents and mutual interference to an adjacent bit line or a second adjacent bit line become conspicuous, and the read characteristic tends to degrade along with advance in micropatterning of the manufacturing process.
Memory cell threshold verification (to be referred to as verify hereinafter) generally performed as an internal operation of the NAND flash memory in a write or an erase operation is none other than a read operation. Hence, not only the read characteristic but also the write and erase characteristics tend to degrade.